Respiratory syncytial virus (RSV) is a significant human pathogen severely impacting neonates and young children, but no vaccine exists to protect this vulnerable population. Furthermore, direct vaccination of neonates is likely ineffective due to the immaturity of their immune system, and neonate immunization is potentially unsafe. The current view is that maternal vaccination is the best and safest approach to protection of neonates through the passive transfer of maternal neutralizing antibodies in utero to the fetus after maternal immunization. However, a complicating issue is that most people have experienced RSV infections during their lifetimes. Thus, any maternal vaccine must induce high levels of protective antibodies in the presence of pre-existing, but very poorly neutralizing, anti-RSV antibodies. In our STTR phase IIa project, we tested our novel RSV VLP vaccine candidates in pregnant cotton rats, a surrogate human model, to determine the effect of prior RSV exposure on induction of protective immune responses in these animals and to assess maternal antibody transfer, protection, and safety in offspring of the immunized dams. Our results showed that a single VLP immunization of RSV primed cotton rats stimulated high titers of NA. Furthermore, VLP immunization of dams protected their offspring from RSV challenge and decreased pathology in pups' lungs compared to pathology observed after RSV infection of offspring of unimmunized dams. We also found that maternal immunizations with VLPs containing different versions of the pre-fusion F protein varied significantly in the extent of protection of offspring of vaccinated dams. We identified one version of pre-fusion F VLPs that improved protection of offspring ten-fold compared to the originally tested pre-F VLP. It is our goal to move our VLP vaccine candidates toward clinical trials as a maternal vaccine. To this end, we propose three specific tasks using cotton rats as human surrogates. Task 1: Develop protocols and processes for production and purification of cost effective GMP VLP vaccine candidates on a large scale. Task 2: Refine parameters of protection of neonates after maternal immunization. Task 3: Assess and improve protective responses in mothers post-delivery.